Materials may be characterized in terms of bulk properties and surface properties. The overall properties of a material are controlled in significant part by the surface properties and the bulk properties of the material. The surface properties of a material are largely controlled by the surface chemistry and surface structure of the material. The bulk properties of a material are largely controlled by the bulk chemistry and bulk structure of the material. It is sometimes desirable to modify the surface chemistry and/or surface structure of a material in order to produce certain surface properties. In addition, it is sometimes desirable to modify the bulk chemistry and/or bulk structure of a material in order to produce certain bulk properties.
Hair and skin are physiological materials of particular interest in terms of surface and/or bulk modification. Hair and skin are exposed to a variety of chemical and physical environments. For example, common hair care practices often include one or more of washing, blow drying, brushing, coloring, perming, relaxing, styling, and the like. These activities repeatedly expose hair to mechanical and chemical factors that may result in the loss of the natural luster and texture that characterizes healthy hair. Moreover, environmental factors may add to these effects and substantially contribute to weathered or damaged hair. Skin also suffers from surface damage as a result of similar mechanical, chemical and environmental factors. Acute damage to the surface of hair and skin may build over time, resulting in chronic damage.
Hair is naturally protected from mechanical, chemical, and environmental mediated damage by the fiber cuticle surface membrane (“FCSM”). The FCSM comprises the outermost surface layer of hair fibers and includes protein and lipid components. The FCSM functions as a highly resistant, hydrophobic, surface-protective barrier to mechanical, chemical and environmental factors that would otherwise substantially contribute to hair damage. The FCSM comprises a surface lipid mono-layer, sometimes referred to as the F-layer, covalently bound to an underlying layer of heavily cross-linked keratinous protein, sometimes referred to as the epicuticle. The F-layer comprises predominately fatty acids such as 18-methyl-eicosanoic acid (“18-MEA”) bound to the epicuticle through thioester linkages formed between the thiol groups on the cysteine residues in the keratin and other proteins in the epicuticle and the carboxyl group on the 18-MEA or other fatty acid. The F-layer gives hair fibers a hydrophobic surface, which in part facilitates the shiny luster, silky texture and smoothness of healthy hair.
Skin is naturally protected from mechanical, chemical, and environmental mediated damage by the stratum corneum. The stratum corneum is the outermost layer of epidermis. The stratum corneum comprises lipid-depleted keratinous cells embedded in a lipid-rich interstitium comprising keratin, fatty acids and ceramides. The fatty acids, ceramides and other lipid components of the stratum corneum are thought to be covalently attached to the proteinaceous components through ester and thioester linkages in a manner similar to the covalent attachment of the F-layer to the epicuticle in hair. The stratum corneum functions to prevent percutaneous moisture loss, regulate percutaneous absorption, and provide a physiologic barrier to protect the lower layers of the epidermis.
Despite differences in microstructure, the F-layer and the stratum corneum both possess similar protective functions for hair and skin respectively. However, mechanical, chemical and environmental factors may result in loss of at least a portion of the F-layer and the stratum corneum. For example, during permanent hair coloring, the combinations of hydrogen peroxide, ammonia and high pH may remove at least a portion of the protective F-layer, allowing for additional oxidation of the underlying hair surface, which may cause irreversible physiochemical changes in the hair fibers. Repeated colorings may cause the F-layer to completely disappear from the surface of hair fibers. As a result, the previously hydrophobic hair fiber surfaces may become hydrophilic because the keratinous epicuticle is exposed to the surface when the F-layer is lost. The natural protective and lubricating properties of the hair fiber surface are consequently diminished, and hair may feel dry, rough, frizzy, become difficult to brush and/or detangle, appear duller and less colorful, possess increased levels of static, and become substantially more susceptible to additional damage due to other mechanical, chemical and environmental factors.
The stratum corneum is similarly susceptible to damage mediated by mechanical, chemical and environmental factors. For example, during the winter months in relatively cold climates, lower humidity levels, low temperatures, and high winds may contribute to xerosis (dry skin), characterized, for example, by redness, itchiness and/or flaking. Damaged skin is substantially more susceptible to further damage, which may transform an acute problem to a chronic condition.
Damage to the surface portions of these materials may lead to damage to the underlying bulk portions of the materials. This may ultimately result in substantial, and perhaps irreparable, damage to these materials. A variety of mechanical, chemical and environmental factors may contribute (solely or collectively) to hair and/or skin damage. For example, excessive exposure to sunlight, exposure to chlorine in pool water (and to a lesser extent in the water provided by municipal supply), exposure to other forms of water pollution, exposure to various forms of air pollution, frictional interactions between hair fibers, and frictional interaction between hair fibers or skin and other surfaces may contribute to hair and skin damage.
Accordingly, there exists a need for compositions and methods to compensate for F-layer and stratum corneum loss from hair fibers and skin, respectively, that provides a durable conditioning and protective benefit. Covalent modification of the surface properties of damaged hair and skin is one example of such an approach. There is also a need to protect, repair, and/or strengthen these materials. Modification of the surface of a material by locally forming an active material on the material surface by reacting one or more active components to create covalent bonds between the one or more active components and modification the bulk of a material by forming active material in a similar manner within the bulk of the material are promising approaches.